This invention relates to a new and improved system for the remote measurement of wind velocity and direction. The new system is particularly suited for making accurate measurements of vertical wind velocity, and for positioning in various locations, the positioning of the various components being highly flexible.
Doppler-acoustic radar has been shown to provide a means for measuring the wind velocity from a remote location. Briefly stated, the measurement is based upon the scattering of acoustic waves by natural fluctuations in the atmosphere. The motion of the air at the scattering point, i.e., the wind, imposes a change in the frequency of the acoustic wave, known as the doppler shift. Coherent processing of the received signal yields a measurement of the doppler shift, from which one component of the wind (determined by the geometrical configuration of the transmitter and receiver) can be calculated. The vector wind can be determined by utilizing different transmission paths to measure independent components.
A configuration that satisfies the requirement for full wind vector determination is shown in U.S. Pat. No. 3,889,533, and the disclosure of that patent is incorporated herein by reference. Specifically, a transmitter antenna produces a vertically-directed beam of acoustic waves. Three receiver antennas are placed on a circle, centered at the transmitter antenna, uniformly spaced at 120.degree. separations. This symmetric configuration yields relatively simple equations for the three wind components u, v and w (respectively directed eastward, northward and upward), and has the additional advantage that the time of arrival of a pulse scattered from a given altitude is the same at all three receivers.
The symmetric configuration described in the patent and its associated signal processing suffer however from two significant shortcomings. One is that the vertical component of the wind occupies a special position of sensitivity compared with the two horizontal components. At low altitudes and also at higher altitudes when averaged over minutes or more, the vertical component of the wind tends to be much smaller than commonly encountered horizontal winds. A given error in measuring the vertical component tends therefore to be relatively more serious. Further, the component of wind that is measured by a receiver antenna displaced from the transmitter antenna is in the direction of the bisector of the angle formed at the scattering point by the rays of the two antennas. This component can be seen to be typically around 20.degree. (or even less) from the vertical for the higher measurement altitudes. The measured component thus weights the vertical components more strongly than it does the horizontal component, and any error in the vertical component is magnified in computing the horizontal components. In both respects then, an accurate measurement of the vertical wind component is desirable.
The second shortcoming of the perfectly symmetric configuration is that it limits the flexibility in siting the installation. Looked at the other way, a user may be unable to or not want to deploy the antennas in a symmetric manner because of terrain variations, structures and other obstructions, or for other reasons of necessity or convenience.
Accordingly, it is an object of the present invention to provide a new and improved wind velocity measuring system which produces a more accurate measurement of the vertical component of wind velocity. A further object is to provide a new and improved wind velocity measuring system in which the antenna configuration is not a limiting factor, permitting placing of receivers at various elevations, distances and angles with respect to the transmitter. An additional object is to provide new and improved signal processing with timing control for the signals from the various receivers.
These and other objects, advantages, features and results will more fully appear in the course of the following description.